Spinal cord injury (SCI) in humans leads to permanent paralysis because injured nerve fibers (axons) do not regenerate. One reason for this is the secretion of chondroitin sulfate proteoglycans (CSPGs) by cells near the injury. CSPGs bind to receptors of the protein tyrosine phosphatase (RPTP) family, PTP? and LAR, and inhibit axon growth. Regeneration after SCI is more successful in lampreys, but some identified reticulospinal (RS) neurons are bad regenerators, and when their axons are injured, these undergo very delayed programmed cell death (apoptosis). We will determine whether, acting through PTP? and/or LAR, CSPGs both inhibit true axon regeneration (as opposed to sprouting by uninjured axons) and trigger retrograde apoptosis after SCI. We will test whether digesting CSPGs with chondroitinase (ChABC) increases survival of spinal-projecting neurons and/or regeneration of their axons, and whether adding extrinsic CSPGs has the opposite effects. Evidence in vitro suggests that CSPGs, acting through LAR activate the small GTPase RhoA and inactivate Akt. Both signals have downstream effects that could inhibit axon growth. By blocking RhoA synthesis or activation and observing the effect on Akt activity, we will determine whether the effect on Akt is downstream of RhoA, or whether the two pathways are triggered independently, perhaps by different RPTPs. RhoA synthesis will be inhibited with morpholino antisense oligonucleotides (MOs) delivered to RS neurons retrogradely from the injury site. Activation of RhoA will be blocked with C3 transferase. The effect on apoptosis markers and axon regeneration will be determined. Synthesis of the RPTPs also will be inhibited with MOs, to determine which receptor mediates which of the negative effects of CSPGs. To test whether effects in lampreys also apply to mammalian neurons, we will perform parallel experiments on postnatal and adult mammalian primary neuronal cultures. We also will test in a mouse optic nerve crush model, whether genetic knockdown or pharmacological inhibition of RPTPs reduces retrograde neuronal death and enhances axonal regeneration in retinal ganglion cells in vivo. Understanding the intracellular pathways that mediate the inhibitory effects of CSPGs on cell survival and axon regeneration could lead to development of therapies for human SCI.